Root mean square voltage regulator

ABSTRACT

A regulating power supply circuit in which the power supplied at the output is controlled by controlling the firing angle of a thyrister switching device in response to changes in a hybrid signal which is indicative of the difference between the RMS value of the output voltage and a desired RMS value. The hybrid signal is formed by summing a feedback current proportional to the absolute load voltage with a reference current proportional to the firing angle of the thyrister on a summing capacitor. Since for any given RMS voltage, the value of the average load voltage varies linearly with the conduction angle over a large range of conduction angles, the phase angle proportional reference current source is utilized to relate the absolute output voltage with the RMS voltage linearly over a wide range of input voltages.

United States Patent [191 Van Cleave 11] 3,746,970 [451 July 17,1973

[75] Inventor: George William Van Cleave,

Lexington, Ky.

[73] Assignee: International Business Machines Corporation, Armonk, NY.[22] Filed: Apr. 13, 1972 [21] Appl. No.: 243,724

3,593,112 7/l97l Coats et al 323/24 Primary Examiner-A. D. PellinenAttorney-John W. Girvin, Jr. et al.

[57] ABSTRACT A regulating power supply circuit in which the powersupplied at the output is controlled by controlling the firing angle ofa thyristor switching device in response to changes in a hybrid signalwhich is indicative of the difference between the RMS value of theoutput voltage and a desired RMS value. The hybrid signal is formed bysumming a feedback current proportional to the absolute load voltagewith'a reference current proportional to the firing angle of thethyrister on a summing capacitor. Since for any given RMS voltage, thevalue of the average load voltage varies linearly with the conductionangle over a large range of conduction angles, the phase angleproportional reference current source is utilized to relate the absoluteoutput voltage with the RMS voltage linearly over a wide rangeof inputvoltages.

3 Claims, 3 Drawing Figures PAIENTEIIJULI .1 I975 TRIGGER THYRISTORGENERATOR 1 ROOT MEAN SQUARE VOLTAGE REGULATOR BRIEF BACKGROUND OFTHE-INVENTION 1. Field This invention relates to a power regulatingdevice and more particularly to a circuit for precisely controlling at aconstant level the amplitude of the root mean square voltage supplied toa load from an A.C. power source.

2. Description of the Prior Art Various voltage regulator devices havebeen constructed to control the power output supplied to a load. Thesedevices include power invertor supplies, ferro resonant supplies, andcontrol circuits adapted to coact with bi-directional current conductingsemiconductor devices. The former devices exhibit large physical sizecharacteristics and furthermore are dependent upon a high degree ofregulation of the input voltage source in order to provide a relativelyconstant RMS voltage output. Further, when the amplitude of thecommercial A.C. voltage supply is changed (as, for example, from countryto country), it is necessary to employ different transformers and/orimpedance networks to provide the desired output RMS voltage.

Various control circuits adapted to co-act with bidirectional currentconducting semi-conductor devices such as thyristers have been proposedwhich will effect a high degree of root mean square voltage regulation.The control circuits generally utilize a feedback signal which isproportional to the average load voltage and weight that signal in amanner to obtain an approximation of the RMS output voltage. Thisfeedback signal then controls the degree of conduction of thebidirectional current conducting device thereby causing the device tomaintain a relatively constant RMS voltage output. An example of such avoltage regulator is described in U.S. Pat. No. 3,532,855 assigned tothe assignee of the present invention. In the patented device, theaverage voltage proportional signal is related to the RMS voltage by acompensating resistor having a current therein proportional to the peakvalue of the input voltage, Various other circuits have also utilizedthe peak value of the input voltage to compensate the out put absolutevoltage signal to obtain, an approximation of the RMS voltage. In eachof these prior devices, the approximation between the output absolutevoltage and the RMS voltage is maintained only over a limited inputvoltage range. That is, large variations in input voltage effectvariations in the output RMS voltage. Such variations are highlysignificant when the output RMS voltage is converted into light energyto be utilized in an environment, such as a copying machine, whereinslight variations in light levels effect large variations in output copyquality.

SUMMARY In order to overcome the above noted shortcomings of the priorart and to provide an RMS voltage regulator which is physically smalland which provides a highly regulated output RMS voltage regardless ofinput voltage variations over a wide range of input voltage variations,the present invention employs a linear method of compensating theabsolute output voltage as supplied by a bi-directional currentconducting device over a wide range of input voltages. Thus, for a givendesired RMS output voltage, a signal representative of the absoluteoutput voltage is fed back to a control circuit and summed with a signalproportional to the conduction angle of the bi-directional currentconducting device. The output signal of the summing device is directlyproportional to the difference between the output RMS voltage of theregulator and a desired RMS voltage over a wide range of input voltagevariations and is utilized to maintain the regulators output at thedesired RMS voltage.

Accordingly, it is the principle object of the invention toautomatically and precisely control the RMS voltage output of a voltageregulator.

It is a further object of the invention to directly sense a signalproportional to the absolute voltage output of the regulator and tocompensate that signal with a second signal proportional to the firingangle of a bidirectional current conducting semi-conductive deviceconnected between the A.C. supply source. and the load to control theRMS voltage output of the regulator.

DESCRIPTION Referring now to the drawings and more particularly to FIG.1 thereof, a schematic equivalent circuit diagram depicting theprinciples employed'in the control circuit of the present invention isshown. The control circuit of the present invention controls a timingcircuit 11 which in turn determines the time from the zero crossing ofthe A.C. line voltage until a thyrister is fired. Firing of thethyrister enables that device to conduct current from a supply to aload. The control circuit is thus utilized to control the firing angleof the thyrister and to thereby maintain the RMS voltage across the loadat a constant. This is achieved by constructing a current balancebetween a current proportional to the average load voltage and astandard current such that an imbalance in current results in thechargingor discharging of capacitor 13. The voltage on capacitor 13 isthe controlling parameter of the variable timing circuit 11.

The implementation of the current balance is achieved by summing threecurrent sources which can further be simplied to two current sources.Initially, a feedback current I proportional to the rectified outputload voltage is combined with a current I 24proportional to theconduction angle of the thyrister. The combined difference current (I-l, is compared with a constant current l to consequently cause thecharging or discharging of the capacitor 13. Providing that the desiredRMS voltage does not require a low or high conduction angle from thethyrister device, a linear relationship exists between the average ofthe feedback current I and the current I, That is for any given RMSvoltage, it can be shown that the value of the absolute voltagecorresponding to the phase con- The foregoing and other objects andadvantages of i trolled sine wave follows the relation: V=AX +B where Xis the conduction angle: V and B are values of absolute voltage; and Ais a constant relating X to B. This linear relationship between I; and 1is thus utilized to provide an excellent degree of RMS voltageregulation which is independent of wide line voltage variations.

Thus, if the feedback current I; exceeds the standard current derivedfrom the currents l and I the capacitor 13 is discharged. This decreasein capacitor voltage results in a longer timing period of the timingcircuit 11 and thus a later firing angle for the thyrister. As theconduction angle is thereby shortened, the rectified absolute voltageacross the load is reduced until the load voltage proportional current Iis brought into balance with the standard current. If the load voltageproportional current I becomes smaller than the standard current, thenthe capacitor 13 is charged. As the capacitor voltage increases, thetiming period of a timing circuit 11 decreases and the thyrister firingis initiated earlier. As the conduction angle is lengthened, the loadvoltage increases until the current balance is restored. In this manner,the control circuit maintains a constant RMS voltage across the load. Ifthe load is resistive, the power in the load will be held constant. TheRMS voltage regulator thus controls the RMS value by utilizing thelinear relationship between the average voltage and the conductionangle. Previous circuits have utilized the peak input voltage to relatethe RMS and average voltage and provide accurate regulation only over asmall input voltage variation range.

The current sources can be constructed in various configurations, thebest regulation being achieved when the phase angle proportional currentand the load voltage proportional current are combined before thecurrent balance is constructed. Thus the current balance is thesummation of a current (I I and a constant current I which yields avirtually constant RMS voltage over a very wide range of input voltagevariations. In an actual circuit design, the current balance isdeveloped by use of transistors, resistors, diodes, and a voltagesource. FIG. 2 is a schematic circuit and block diagram of the RMSvoltage regulator of the present invention utilizing a simplifiedcontrol circuit.

Referring now to FIG. 2 of the drawings, a thyrister 21 is connectedbetween a 1 15 volt A.C. supply in series with a load such as anillumination lamp 23. The thyrister 21 is a silicon bidirectional triodedevice or switch capable of conducting relatively high current in bothdirections and whose time of initial conduction during a half cycle isdependent on when a control voltage is impressed across transformer 25.Once the thyrister 21 is rendered conductive during a half cycle of tthe supply voltage, it will remain conductive until the supply voltageis reversed at the beginning of the next half cycle of the supplyvoltage. The overall arrangement is that bycontrolling the firing angleof the thyrister 21, the root mean square voltage across theillumination lamp 23 is acurately regulated.

The firing angle of the thyrister 21 is controlled by feeding back theinstantaneous value of the voltage across the lamp over feedback leads27, 29 to the bridge rectifier 31. The output signal of the bridgerectifier 31 thus represents the rectified value of the instantaneousload voltage and is applied to transistor 33.

Transistor 35 has its base electrode clamped to the supply voltage 37 byZener diode 39. Transistor 35 thus supplies a constant current to thesumming capacitor 13 tending to charge the capacitor 13. The currentsupplied by transistor causes the charge on capacitor 13 to graduallybuild-up thereby creating a soft-start" of the voltage regulator. Oncethe charge level on capacitor 13 biases transistors 41 and 42 intoconduction, a current proportional to the voltage on capacitor 13 issupplied to sawtooth capacitor 43. When the charge on this capacitorreaches a pre-determined level, the threshold detector circuit 45provides a signal to the thyrister trigger generator 47 which in turncauses the thyrister 21 to conduct. The charge on the sawtooth capacitor43 is reset each half cycle by the reset switch 49 which is in turnresponsive to the zero crossing detector 51. That is, when the A.C.signal from the 115 volt supply reverses polarity, the zero crossingdetector 51 effects the temporary closure of the reset switch 49providing a discharge path for the capacitor 43. Thus a voltage ramp,having a slope proportional to the voltage on capacitor 13, is producedon capacitor 43 which determines the firing angle of the thyrister.

Whenthe thyrister 21 fires, the switch 53 is closed providing a currentthrough transistor 55 which is subtracted from the feedback currentsupplied by transistor 33. The resultant current is in turn combinedwith the constant current supplied by the transistor 35. [hitially, thevoltage at the base electrode of transistor 41 slowly rises due to thecharge across the capacitor 13 provided by the current from transistor35. This slowly rising voltage causes the sawtooth wave form generatedby capacitor 43 to'have a gradually increasing slope and hence greateramplitude at the completion of each successive half cycle. Eventually,the level of the sawtooth is sufficient to trigger the thresholddetector 45 which in turn causes the thyrister trigger generator toprovide a pulse to the thyrister 21. The thyrister at this time conductsonly during a short final portion of the A.C. half cycle therebyproviding a small amount of feedback current through the leads 27, 29.Additionally, switch 53 is closed during the conduction of the thyrister21 thereby providing a current which is subtracted from the feedbackcurrent. This combined current is subtracted from the constant currentsupplied by the transistor 35 causing the rate of charge of capacitor 13to decrease slightly. However, the voltage on the capacitor 13 continuesto increase until the compensated feedback current is equal to theconstant reference current supplied by transistor 35 at which time thecharge level remains constant. At this point, an equilibrium conditionis maintained since the charging current to the sawtooth capacitor 43 isproportional to the voltage on the capacitor 13.

Referring now to FIG. 3, a circuit diagram of an alternate controlcircuit which may be utilized with the RMS voltage regulator of rhepresent invention is depicted. This control circuit may be used in lieuof the control circuit 57 depicted in FIG. 2 of the drawings. Inoperation, transistor 35 supplies a constant charging current to thecapacitor 13' in a manner analogous to that heretofore described withrespect to the control circuit 57. The current supplied by thetransistor 55' when there is no feedback signal on leads 27, 29 flowsthrough the bottom portion of the bridge rectifier 31 to A.C. ground 59.This current thus has no effect on the charge level of capacitor 13.When a feedback signal is supplied over feedback leads 27-29, thefeedback current supplied at the base electrode of transistor 33 tendsto discharge the charge on capacitor 13' when the feedback currentexceeds the current supplied by transistor 55'. That is, the currentsupplied by transistor 55' subtracts from the feedback current wheneverfeedback current is present. When the feedback current exceeds thatsupplied by transistor 55', it subtracts from the current supplied bytransistor 35. An equilibrium condition then results as described withrespect to FIG. 2 wherein the charge on capacitor 13' achieves aconstant level.

The bridge rectifier 31 acts in an analogous manner to the switch 53 ofFIG. 2 causing a phase angle proportional current as supplied bytransistor 55' to be subtracted from the feedback current. It is to benoted that the circuit of FIG. 3 differs from that of FIG. 2 in that itprevents the current flowing from transistor 55 from affecting thecharge on the capacitor 13 when that current exceeds the feedbackcurrent. It has been found that the linear relationship between thefeedback the build up of charge on capacitor 13. The soft start protectslamp filaments from excessive cold start currents.

A circuit was constructed with the following component values to providean RMS voltage of 85 volts regulated :1 percent from a 115 volt sourcewhich varies percent:

R65-l 5.8K

R67-l 1 8K Cl3'-330 uf Z39-3.6 V nominal T55, T35, T33-IBM 136, 136, IBM194 From the foregoing description of the root mean square voltageregulator, it will be apparent that an improved economical arrangementhas been provided for controlling the root mean square magnitude of theload voltage which is virtually insensitive to changes in the supplyvoltage. As is appreciated by those skilled in the art, various circuitarrangements can be utilized to achieve the invention. For example, asingle current source could be utilized to supply both the phase angledependent current (I as well as the constant current (1 it being onlynecessary to supply both currents to comprising:

switching means switchable from a high impedance condition to a lowimpedance condition in each half cycle connected between said load andsaid alternating current supply;

said switching means having at least one control element for switchingsaid switching means to said low impedance condition when a controlsignal supplied to the control element exceeds a predetermined thresholdlevel;

a feedback circuit connected in circuit with said switching means andsaid load for supplying a first signal indicative of the absolute valueof the output voltage;

circuit means coupled with said switching means for supplying a secondsignal indicative of the duration within each half cycle of the lowimpedance condition of said switching means;

a constant signal source for supplying a constant signal;

summing means for combining at least portions of said first signal, saidsecond signal, and said constant signal for synthesizing a hybrid signalindicative of the difference between the root mean square value of theoutput voltage signal and a desired root mean square value;

and means coupled with said summing means and said control element forsupplying said control signal as a function of said hybrid signalthereby to maintain the' root mean square magnitude of the outputvoltage substantially at a preselected level.

2. The root mean square voltage regulator set forth in claim 1 whereinsaid feedback circuit includes rectifying means connected in circuitwith said switching means and said load for supplying said first signal.

3. The root mean square voltage regulator set forth in claim 1 whereinsaid first signal, said second signal,

and said constant signal being current signals and wherein said summingmeans comprises a capacitor.

1. A root mean square voltage regulator for controlling in each halfcycle the output voltage supplied to a load from an alternating currentsupply, said regulator comprising: switching means switchable from ahigh impedance condition to a low impedance condition in each half cycleconnected between said load and said alternating current supply; saidswitching means having at least one control element for switching saidswitching means to said low impedance condition when a control signalsupplied to the control element exceeds a predetermined threshold level;a feedback circuit connected in circuit with said switching means andsaid load for supplying a first signal indicative of the absolute valueof the output voltage; circuit means coupled with said switching meansfor supplying a second signal indicative of the duration within eachhalf cycle of the low impedance condition of said switching means; aconstant signal source for supplying a constant signal; summing meansfor combining at least portions of said first signal, said secondsignal, and said constant signal for synthesizing a hybrid signalindicative of the difference between the root mean square value of theoutput voltage signal and a desired root mean square value; and meanscoupled with said summing means and said control element for supplyingsaid control signal as a function of said hybrid signal thereby tomaiNtain the root mean square magnitude of the output voltagesubstantially at a preselected level.
 2. The root mean square voltageregulator set forth in claim 1 wherein said feedback circuit includesrectifying means connected in circuit with said switching means and saidload for supplying said first signal.
 3. The root mean square voltageregulator set forth in claim 1 wherein said first signal, said secondsignal, and said constant signal being current signals and wherein saidsumming means comprises a capacitor.